"Everybody" was Christan at the time in the same way that everyone in the past was heterosexual.
Since the cult punished people (either violently, or socially) in societies it had conquered, there's no way of knowing how many people where "believers", or just went along out of fear.
My home country was under oppression from the Christian sect for about a thousand years, and guess what? As soon as it was legal and not punished by society, pretty much everyone turned their back on it, to live normal lives. The only people who actually join the cult are either born into broken cultist families, or struggle with addiction to the point where giving up control of their life seem like the only solution.
Exposing children to religion is abuse and nothing else.